The development of disease modifying agents in Parkinson's disease has rapidly expanded the need for in vivo markers for diagnosis and monitoring disease progression. Dopamine transporter (DAT) imaging offers the promise of an objective measure of dopaminergic degeneration allowing for identification of changes in the brain that occur early in the illness, prior to clinical diagnosis. The primary goal of this project is to examine the sensitivity and specificity of DAT imaging using (3-CIT and SPECT imaging as a diagnostic marker in subjects with suspected PD or PS. Neurologists will identify subjects in whom they have genuine uncertainty regarding diagnosis of PD or PS. The neurologists will be asked to document their 'best guess' diagnosis on a Diagnostic Accuracy Questionnaire at the time of referral. Subjects with suspected PD or PS will be evaluated clinically and with DAT imaging at MNI. The blinded Parkinson's expert will re-examine the subject in 6 months and make a final clinical diagnosis, which will serve as the gold standard diagnosis for each subject. The DAT imaging diagnosis will be compared to the 'gold standard' clinical diagnosis to determine the sensitivity of (3-CIT and SPECT imaging as a diagnostic marker in PD and PS. This project is a crucial step to begin to establish B-CIT and SPECT imaging as an objective diagnostic biomarker prior to definitive diagnosis.